1. Field of the Invention
The present invention relates to a solid-state image sensing device such as a CMOS image sensor that digitalizes and outputs analog pixel signals output from pixels, and an imaging method and an imaging apparatus.
2. Background Art
In the related art, in a CMOS (Complementary Metal-Oxide Semiconductor) sensor as a solid-state image sensing device, image signal processing is performed using a CDS (Correlated Double Sampling) circuit.
For example, Japanese Patents Nos. 3734717 and 3710361 disclose a CMOS sensor that removes noise contained in image signals by passing light reception signals from photodiodes within pixels through analog CDS circuits provided with respect to each pixel column and performs A/D (Analog/Digital) conversion thereon.
However, when the CDS circuits are used in this manner, there is a problem that noise in a streaky fixed pattern occurs due to variations of CDS circuits with respect to each pixel column. Further, there are problems that capacitative elements for holding signal values after CDS processing are necessary and the circuit areas are increased, and analog signals are horizontally scanned at a high speed with a shift register and easily affected by switching noise.
Accordingly, for example, JP-A-2005-328135 proposes that these problems are solved by a column parallel A/D conversion system (hereinafter, referred to as a column AD system).
In the column AD system, A/D converters are provided with respect to each pixel column, and analog signals of the respective pixels are read out together with respect to selected columns into the respective vertical signal lines, and A/D conversion is directly performed thereon. Accordingly, the problems occurring when the above described CDS circuits are used are solved and high-accuracy noise removal can be executed.
Further, in the column AD system, parallel processing with respect to each horizontal row of an image is performed, and it is not necessary to drive horizontal scan at a high-speed frequency. Therefore, there are advantages that A/D conversion can be performed at a vertical low-speed frequency, and the noise component generated in a high-frequency band and the signal component can be easily separated.
In the CMOS sensor employing the column AD system, the pixel signals supplied from the pixels to the A/D converters contain reset components according to a predetermined reference potential and data components according to amounts of received light of the pixels.
Further, a ramp signal (ramp voltage) as a signal that is referred to at A/D conversion of the pixel signals is supplied to the A/D converters.
The ramp signal has a waveform in which the voltage drops with a fixed gradient from a predetermined initial voltage in a period according to the reset components of the pixel signals, and the voltage drops with a fixed gradient from a predetermined initial voltage in a period according to the data components of the pixel signals.